Add 'noop' command

Add a `noop` command that returns the `client.api` object to a callback and allows e.g. to store or print variables.

```
this.demoTest = function (browser) {
  browser
    .noop(function(api){
      console.log('hello from a noop command.');
    });
};
```

Let me know what you think.. 

regards
~david